Fix SHLIB_PREREQS use in contrib, allowing PGXS builds

dblink and postgres_fdw use SHLIB_PREREQS = submake-libpq to build libpq
first.  This doesn't work in a PGXS build, because there is no libpq to
build.  So just omit setting SHLIB_PREREQS in this case.

Note that PGXS users can still use SHLIB_PREREQS (although it is not
documented).  The problem here is only that contrib modules can be built
in-tree or using PGXS, and the prerequisite is only applicable in the
former case.

Commit 6697aa2bc25c83b88d6165340348a31328c35de6 previously attempted to
address this by creating a somewhat fake submake-libpq target in
Makefile.global.  That was not the right fix, and it was also done in a
nonportable way, so revert that.
Remove USE_VPATH make variable from PGXS

The user can just set VPATH directly.  There is no need to invent
another variable.
Revert haphazard pgxs makefile changes

These changes were originally submitted as "adds support for VPATH with
USE_PGXS", but they are not necessary for VPATH support, so they just
add more lines of code for no reason.